Sorry, me again... If your skin has formed a callus around/above something which is painful when probed, it sounds as though your body is trying to isolate it, which means there might be something foreign trapped under there that could have become infected. (Is there any inflammation or swelling, or - warning signal - red streaks emanating from it?) If it IS an infection (and the sharp pain you describe reminds me of one I once had) it needs professional dealing with, and sooner not later!